1. Is it possible to signup for GoPhone and use unlocked iPhone with it?
Absolutely. I use my iPhone 3G under the prepaid Pay-as-You-Go plan. Under 10c-min talk, 20c-message, and 100MB-month data. I get 3G service throughout my coverage area (Washington, DC). [All smatphones that want to use data must participate on one-of-three GoPhone data plans.]
2. Would it be possible to use my existing number and transfer it to AT&T?
Absolutely. I assume that you are, today, under an AT&T postpaid plan. Activation and transfer of your old number should happen simultanously, then.
ASSUMPTION: I assume that you are postpaid, today, because the 3G UMTS Bands serviced by the iPhone (850MHz/1900MHz) are only supported by AT&T in North America, not T-Mobile, which is your other GSM alternative.
If you haven't already done so, I would contact T-Mo customer service to see if they can provide you with an account number for your prepaid line of service. I believe that's what at&t prepaid customers have to do in order to port their number to another carrier, so I presume it would work similarly coming from T:-Mo.
You can port telephone numbers that are assigned to a prepaid line of service, but it takes a little more effort than it does with post paid service.
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.